1. Field of the Invention
The invention relates generally to apparatus and methods for perforation in a wellbore.
2. Background Art
After drilling oil wells are typically protected with steel casing that is secured to the wellbore with cement. In order to establish communication between oil/gas formations and the cased well, perforation guns carrying shaped charges are used. These shaped charges contain explosives. When the explosives are fired, they produce high pressure and high temperature. As a result, the shaped charge liners are shot out as jets that can penetrate the casing and the nearby formation.
There are two basic types of shaped charges for perforating applications, one type is big hole charges which can make large holes on the casing and relatively shallow penetration in the formation rock. Such shaped charges are typically used when big holes or big area of flow are needed, such as in sand control applications. The other type is deep-penetrating charges which can make relatively small holes on the well casing, but they can penetrate deep into the formation rock. The deep-penetrating liner jets can shoot through the damaged zones from drilling and significantly enhance the well productivity. The deep-penetrating charges are typically used in natural completion applications.
In addition to the two basic types of shaped charges described above, there are also encapsulated shaped charges, which are exposed to wellbore fluids directly and, therefore, they are sealed individually with a cap. The encapsulated shaped charges produce more debris than the same size charges carried by a hollow gun, although the encapsulated shaped charges do make bigger holes on the casing and deeper penetration into the formation rock, as compared to the non-encapsulated types.
In addition to different types of shaped charges, the dynamic pressure generated during gun detonation has also proved to be critical for well productivity. Proper manipulation of the dynamic pressure can significantly enhance well productivity. For example, by using reactive material in the shaped charge cases, the explosive pellets, and/or the liners, the heat generated from these reactive materials during detonation can have an impact on the wellbore pressure. In addition, the charge performance could also be increased by putting more of the energy into the shaped charge jets.
After firing, debris from the shaped charges and guns will be left inside the guns, wellbore, and/or formations. For example, the debris from the shaped charge jets may be left in the tunnels that were generated by the jets. This debris can clog the pores and reduce the productivity of the well, leading to big loss.
To avoid some of the problems associated with shaped charge debris, various shaped charge designs have been proposed. For example, there are charges designed to reduce the shaped charge case debris, e.g., OrientX™ charge, the 3 on a plane packing design of the big hole charges, e.g., PF4621, 6618, 7018, etc. Similarly, other designs are to reduce liner debris, e.g., powdered-metal liners, dual layer metal (zinc and copper) liners for PowerFlow™ charges. In addition, underbalanced perforating system such as PURE™ is widely used to manipulate the wellbore dynamic pressure to clean the perforating tunnels.
Even though these prior art methods are effective in reducing problems associated with shaped charge debris, there remains a need for ways to avoid or minimize debris-caused problems after perforation.